3.3 Outbreak Investigation Protocols & Epidemic Curves

3.3 Outbreak Investigation Protocols & Epidemic Curves

A healthcare-associated outbreak is defined as an increase in the occurrence of a disease or pathogen above its expected baseline (endemic) level within a specific facility, unit, or patient population. When an outbreak occurs, Infection Preventionists must act rapidly as epidemiologic investigators to identify the source, stop transmission, protect patients and staff, and implement long-term preventive controls.

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The 10-Step Outbreak Investigation Process

The CDC and APIC outline a standardized 10-step framework for conducting healthcare outbreak investigations. While presented sequentially, steps often occur simultaneously in practice.

1. Confirm Diagnosis and Verify Outbreak Existence

Verify laboratory reports and diagnostic findings. Compare current case numbers against historical baseline data (prevalence or incidence) to confirm that a true cluster exists rather than a random baseline statistical fluctuation.

2. Assemble Investigation Team and Notify Key Stakeholders

Establish an outbreak response team led by the IP, infectious disease physicians, microbiology director, nursing leadership, environmental services, and administration. Notify local public health departments if the pathogen is reportable.

3. Establish a Working Case Definition

Formulate objective, standardized criteria to classify cases. Case definitions must specify:

• Person: Age, patient type, clinical diagnosis
• Place: Specific unit, wing, operating room, or building
• Time: Onset date window
• Clinical / Laboratory Criteria: Specific symptoms, isolation of specific pathogen, resistance phenotype, or molecular subtype

Cases are categorized as Confirmed (laboratory verified with matching molecular strain), Probable (typical clinical symptoms and strong epidemiologic link), or Suspected (unverified clinical symptoms meeting basic criteria).

4. Conduct Active Case Finding and Construct Line Listing

Proactively search for additional cases across clinical units using active surveillance. Compile all identified cases into a line listing spreadsheet, documenting patient identifier, age, sex, room location, admission date, symptom onset date, underlying diagnoses, invasive procedures, staff assignments, and microbiological results.

5. Perform Descriptive Epidemiology

Analyze line listing data by time, place, and person:

• Time: Construct an epidemic curve (Epi Curve) plotting case onset dates.
• Place: Create a spatial spot map of the clinical unit or operating suite to identify geographic clustering.
• Person: Summarize demographic features, underlying comorbidities, device exposures, and shared healthcare personnel.

6. Formulate Hypotheses

Based on descriptive epidemiologic patterns and known pathogen transmission mechanisms, formulate hypotheses regarding the likely source, reservoir, mode of transmission (contact, droplet, airborne, common vehicle), and risk factors.

7. Evaluate Hypotheses via Analytical Studies

Test hypotheses using quantitative epidemiological study designs:

• Case-Control Study: Compares infected cases to non-infected controls from the same unit. Calculates the Odds Ratio (OR). Ideal when investigating rare outcomes or when a well-defined cohort cannot be established.
• Cohort Study: Compares attack rates between exposed and unexposed patient groups. Calculates Relative Risk (RR). Ideal when an entire exposed population is known (e.g., all patients undergoing surgery in a specific suite on a single day).

8. Refine Hypotheses & Conduct Environmental / Laboratory Investigations

Perform targeted environmental sampling (e.g., testing hospital water outlets, high-touch surfaces, or medication vials) and advanced microbiological molecular typing—such as Whole Genome Sequencing (WGS) or Pulsed-Field Gel Electrophoresis (PFGE)—to confirm strain identity between environmental reservoirs and clinical isolates.

9. Implement Immediate Control Measures

CRITICAL RULE: IPs must never delay control measures while awaiting analytical study results or molecular laboratory typing! Immediate interim interventions—such as isolation precautions, dedicated staff assignments, equipment restriction, product quarantine, and enhanced environmental cleaning—must be instituted as soon as transmission is suspected.

10. Communicate Findings and Document Final Report

Prepare a formal written outbreak report detailing epidemiology, root causes, control interventions, and policy revisions. Disseminate recommendations to facility leadership, quality committees, and public health authorities.

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Epidemic Curves (Epi Curves): Construction & Interpretation

An Epidemic Curve (Epi Curve) is a visual histogram plotting the number of new outbreak cases ($Y$-axis) against time of illness onset ($X$-axis). The time interval on the $X$-axis is typically set to approximately one-fourth to one-half of the pathogen's average incubation period.

Epi curves provide key insights into the nature of exposure and mode of transmission:

1. Point-Source (Common Vehicle) Curve

• Pattern: Steep, rapid upward slope reaching a single high peak, followed by a gradual or rapid decline within a single incubation period.
• Indicates: All cases were exposed simultaneously to a single contaminated source over a brief time window (e.g., a contaminated batch of IV flush solution or foodborne exposure at a hospital event).

2. Continuous Common-Source Curve

• Pattern: Rapid initial rise in cases, followed by a prolonged plateau phase reflecting sustained case counts.
• Indicates: Ongoing continuous exposure to a contaminated environmental reservoir over an extended period (e.g., persistent contamination of a hospital potable water line or ice machine).

3. Propagated (Person-to-Person) Curve

• Pattern: A series of progressively taller peaks separated by intervals approximately equal to one incubation period.
• Indicates: Secondary transmission spreading from person to person (e.g., viral gastroenteritis [Norovirus], Influenza, or scabies).

4. Intermittent Common-Source Curve

• Pattern: Irregular, recurring peaks with variable intervals between clusters.
• Indicates: Intermittent exposure to a contaminated source (e.g., an improperly reprocessed endoscope used periodically for elective procedures).

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Differentiating True Outbreaks from Pseudo-Outbreaks

A pseudo-outbreak (or false cluster) is defined as an artifactual rise in positive laboratory reports or clinical cases without actual patient infection or clinical illness.

Common Causes of Pseudo-Outbreaks

1. Environmental / Equipment Contamination: Contamination of automated endoscope reprocessors, tap water, blood gas analyzers, or collection tubes leading to false-positive cultures (e.g., environmental non-tuberculous mycobacteria like Mycobacterium abscessus isolated from bronchoalveolar lavage specimens).
2. Laboratory Artifacts: Changes in microbiology testing methodology, introduction of higher-sensitivity PCR assays, or cross-contamination during specimen processing in the lab.
3. Surveillance Bias: Sudden increased clinician sampling or changes in diagnostic ordering patterns.

Key Distinguishing Features

Patients involved in pseudo-outbreaks typically show no clinical signs of systemic illness (no fever, leukocytosis, or organ dysfunction), yet have positive cultures. Recognizing pseudo-outbreaks prevents unnecessary antibiotic administration, invasive procedures, and costly facility closures.